Early in the development and testing of optical waveguide fiber, the impact of external forces on waveguide properties was studied and quantified. Both large and small radius bends (macrobending and microbending respectively), relative to the wavelength of light propagated, can cause power to be coupled out of the waveguide, thereby increasing attenuation. In the case of multimode waveguides, bandwidth can be increased or decreased by bending the waveguide. In the case of single mode fiber, bending can change the waveguide cut-off wavelength and can introduce local changes in refractive index, an effect called stress optic birefringence. Stress optic birefringence in turn can cause increased coupling between the two polarization modes propagated in the fiber. For applications, such as gyroscopes and other sensors, where maintenance of a particular polarization is of critical importance, bend induced stress optic birefringence can essentially render the waveguide inoperative. The need to prevent or limit this detrimental behavior is therefore clear.
Methods for insulating or isolating the waveguide from external forces, which can produce detrimental bending effects, typically involve waveguide coatings having a low elastic modulus, or cable designs which do not allow bending to occur or do not transmit bending to the encased waveguide. The combination of low modulus coatings with protective cable designs has produced waveguide cable with acceptable performance over a wide range of applications and environmental conditions.
However, there are difficulties and costs which accompany these solutions to the waveguide bending problem. In the case of coatings, the application process must yield a coating of uniform thickness and density to prevent the coating itself from inducing waveguide bending. Manufacturing cost and throughput are thereby adversely affected. Also, for essentially all coatings, the coating modulus changes with temperature, which means that the bend less resistance of the waveguide/coating combination can vary with temperature. A second, higher modulus coating applied over the low modulus coating is usually required to protect the waveguide from abrasion and thereby maintain strength. This second coating must bond to the first coating and be applied uniformly and homogeneously, so that the second coating provides protection without inducing waveguide bending. Again, cost and manufacturability are adversely affected.
Cable designs which resist bending or protect the fiber from external forces, which may induce bending, in general require additional components such as slotted cores or plastic tubes. Thus, these designs are more costly and usually result in a cable having increased diameter and weight.
Furthermore, there are applications where tolerances on waveguide properties, environmental requirements or cable size and weight requirements are such that the coating and cable designs described above do not insure adequate performance.
The present invention is distinct from the prior art, as described below.
Miller, U.S. Pat. No. Re. 28,664 describes an optical waveguide fiber with a cross section having spaces between a central light carrying member and a peripheral supporting structure. The central member is attached to the peripheral structure by means of two or more thin films extending therebetween. The central member, the supporting structure and the thin films are all fabricated from the same material. The air or vacuum surrounding most of the central member acts as the lower index "cladding" required to confine light to the central member. The problem addressed in U.S. Pat. No. Re. 28,664 is that of constructing an optical waveguide suitable for telecommunications, i.e., low attenuation. Bend less resistance is not addressed.
Both Hicks, U.S. Pat. No. 4,630,889 and Hicks, U.S. Pat. No. 4,634,218 describe polarization maintaining waveguides with a cross section having spaces between a central member and a surrounding structure. As in U.S. Pat. No. Re. 28,664, the entire central member, i.e., the light carrying member, is composed of the same material. The index difference, required to define a light guiding core region, is achieved by holding the center member or members under stress. In general, the relative thermal expansion coefficients and geometry of the central member or members, compared to the surrounding structure, are chosen such that, in final configuration, the surrounding structure of the waveguide places a compressive load on the central member or members. The contours of the resulting stress field define the refractive index gradient and thus establish the polarization maintaining feature. Any bend less resistance of this waveguide is due to directional compressive loading, rather than isolation of the central structure from its surroundings.